Add Payment to account

There might be an issue trying to do it through the app. Try a different method by logging in from any web browser to https://appleid.apple.com and edit your billing information.

If you're using an unsupported payment card, you might need to add a cash balance to your account first:

https://support.apple.com/en-us/HT208687

See also here for changing payment methods: https://support.apple.com/en-us/HT201266